The reference voltage source is an extremely important component of an analog integrated circuit. For an analog-to-digital/digital-to-analog conversion circuit, a low voltage drop linear regulator circuit, and various sensors (for example, a temperature sensor, a pressure sensor and the like), the precision of the reference voltage determines the maximum precision of the circuits. The reference voltage source is mainly categorized into a buried Zener reference voltage source, an extract implantation junction field effect transistor (XFET) reference voltage source, and a bandgap reference voltage source. The bandgap reference voltage source is categorized into a bipolar bandgap reference voltage source and a sub-threshold complementary metal oxide semiconductor (CMOS) bandgap reference voltage source.
An ideal reference voltage source is capable of providing a voltage irrelevant to the voltage of the power supply, the temperature, and the process. However, in practice, such factors as discreteness of the process may cause a significant impact on the precision of the reference voltage. In this case, the reference voltage needs to be adjusted. A traditional adjusting method is generally adjusting the resistance or the current in the circuit of the reference voltage source, and adjusting the value of the reference voltage by disconnecting or connecting some resistors or by reducing or increasing some currents. Adjustment of the voltage in the reference voltage source may be implemented by adjusting the temperature coefficient (TC). The temperature coefficient represents a proportion of voltage variation of the reference voltage within a specific temperature variation range (the lowest temperature TL to the highest temperature TH). The formula for calculating the temperature coefficient is Equation (1):
                    TC        =                                                                              V                  REF                                ⁡                                  (                  max                  )                                            -                                                V                  REF                                ⁡                                  (                  min                  )                                                                                                      V                  REF                                ⁡                                  (                  average                  )                                            ⁢                              (                                                      T                    H                                    -                                      T                    L                                                  )                                              ×                      10            6                    ⁢                                          ⁢                      ppm            /            °                    ⁢                                          ⁢                      C            .                                              (        1        )            
In the above equation, (TH−TL) is a temperature variation range, VREF(max) is a maximum value of the reference voltage within the temperature variation range, VREF(min) is a minimum value of the reference voltage within the temperature variation range, VREF(average) is an average value of the reference voltage within the temperature variation range, and ppm/° C. is the unit of the temperature coefficient.
Adjustment of the temperature coefficient involves adjustment of the temperature compensation proportion of the reference voltage, and is generally related to the structure of the reference voltage source. In the prior art, for different reference voltage sources, different adjusting methods may be used to adjust the temperature coefficients, thereby implementing calibration of the reference voltage. For example, with respect to a traditional bipolar bandgap reference voltage source, the emitter-base voltage of a PNP transistor has a negative temperature coefficient, whereas a difference of the emitter-base voltages of two PNP transistors operating in different current densities has a positive temperature coefficient. Therefore, theoretically a reference voltage irrelevant to the temperature may be obtained by means of addition of these two voltages at different proportions.
Referring to FIG. 1, a schematic structural diagram illustrating a circuit of a typical Banba reference voltage source in the prior art is given. R1, R2, R3, and R4 are resistors with adjustable resistance, a current ratio of a p-type metal-oxide-semiconductor (PMOS) transistor P1 to a PMOS transistor P2 is 1:1, a current ratio of the PMOS transistor P2 to a PMOS transistor P3 is 1:1, an emitter junction area ratio of a PNP transistor P4 to a PNP transistor P5 is 1:n, and a voltage of the emitter-base of the PNP transistor P4 is VEB. In this case, the reference voltage is represented by Equation (2):
                              V          REF                =                                            R              3                                      R              2                                ⁢                      (                                          V                EB                            +                                                                    R                    1                                                        R                    2                                                  ⁢                                  V                  T                                ⁢                ln                ⁢                                                                  ⁢                n                                      )                                              (        2        )            
In the above equation, VT=kT/q, wherein k is a Boltzmann constant, q is a quantity of electron charge, and T is a Kelvin temperature. According to Equation (2), the first-order temperature coefficient is related to R1 and R2. Therefore, by adjusting the value of R2 or R1, the proportion of the kT/q (that is, VT) is changed, to achieve adjustment of the first-order temperature coefficient, thereby implementing calibration of the reference voltage. The calibration precision depends on the design indicators of the reference voltage source. However, such method for adjusting the temperature coefficient is subject to some limitations. In one aspect, such adjusting method is dependent on the specific structure and implementation manner of the reference voltage source, and has no universality, which is thus not applicable to a reference voltage source without resistor, for example, a buried Zener reference voltage source, an XFET reference voltage source, and a sub-threshold CMOS bandgap reference voltage source. In addition, since the sub-threshold CMOS bandgap reference voltage source practically has smaller current and component size, adjustment of the reference voltage is hard to implement. In another aspect, such adjusting method is capable of only implementing adjustment of the first-order temperature coefficient. However, with respect to a bandgap reference voltage source, the second-order temperature coefficient of about 10 ppm/° C. is generally included; or with respect to a system requiring a precision of above 12 bits, a relatively complicated high-order temperature compensation circuit needs to be configured. Such high-order compensation circuit generally induces component mismatch to some extent, thereby causing offset of the reference voltage.